The present invention relates to a device for measuring the deformations of a diaphragm, those deformations resulting from a physical phenomenon such as a pressure variation on both sides of this diaphragm or an acceleration.
The invention more particularly relates to such measuring devices wherein the detection of deformations is ensured by piezo-resistive sensors constituted by a thick layer (having a thickness of about 10 micrometers) of a piezo-resistive material, for example through silk screening or ink jet.
The state of the art in this field will be reminded in relation with FIGS. 1A-1D.
A conventional diaphragm sensor comprises a flexible diaphragm 1 embedded on its circular periphery in a support 2. This diaphragm is, for example, made in Al.sub.2 O.sub.3 ceramics, as well as its support. Devices of this type are generally used to measure pressures for detecting possible pressure differences on both sides of the diaphragm.
Due to pressure, the diaphragm is deformed and the piezo-resistive sensors (single or double) are arranged on the diaphragm for measuring the deformations thereof.
According to a conventional implementation, four sensors are provided for; two, r1 and r3, being arranged at the periphery and the other two, r2 and r4, being arranged close to the middle of the diaphragm. Those four sensors are on the same diaphragm surface because it is generally considered that this simplifies manufacturing and permits to better adjust the four sensors to same values especially due to the fact that they will result exactly from the same deposition and same manufacturing process.
A top view shows such an exemplary sensor in FIG. 1C. A thick layer of piezo-resistive substance 4 is arranged between two previously deposited metallizations 5 and 6. The size of a sensor is about one millimeter. This is an example of a particularly simple sensor but those skilled in the art will be able to use various types of more sophisticated sensors.
Sensors are generally mounted in a Wheatstone bridge as illustrated in FIG. 1D. The two sensors r1 and r3 arranged at the diaphragm periphery are facing each other in the bridge as well as the two sensors placed in the centre. Thus, at rest, since the four sensors r1-r4 have the same resistance r0, a voltmeter V arranged in a diagonal of the bridge, the other diagonal of which is fed by a voltage E, will see a null voltage.
During deformation, the resistances of r1 and r3 will vary in a first direction and the resistances of sensors r2 and r4 will vary in opposite direction. If .DELTA.r designates the resistance variation of a sensor, the detected voltage V will be: EQU V=VO+EO [.DELTA.r2-.DELTA.r1+.DELTA.r4-.DELTA.r3]/4r0, (1)
VO being an error voltage, normally constant, that can be rendered very low by adjusting the sensor size of the piezo-resistive layer portions and/or by providing externally adjustable resistors that may be formed at the diaphragm periphery at positions not submitted to stresses and accessible once the diaphragm has been fixed on its support.
Thus, at first sight, such a structure gives a satisfactory result.
To obtain pressure measurements independent of temperature variations, a reference temperature is chosen for carrying out measurements. However, a hysteresis phenomenon associated with thermal variations occurs. During thermal cycles, residual stresses appear on resistances, those stresses being different according as this reference temperature is reached from a high temperature value or from a low temperature value. Moreover, this hysteresis will not be the same for peripheral sensors as for central sensors. This is due to the fact, known per se, that for peripheral sensors and central sensors, the ratio between the transversal sensibility and the longitudinal sensibility is not the same. Thus, considering again the above equation (1), in addition to resistance variations associated with deformations, there will be an additional resistance variation associated with the thermal history of the sensor. This difference will not be eliminated due to the fact it has the same sign for sensors r2 and r4 centrally arranged, on the one hand, and for sensors r1 and r3 arranged at the periphery, on the other hand. This hysteresis is no longer acceptable when it is desired to reach an accuracy better than 1%.
To palliate this drawback, various approaches have been devised in the prior art, especially as regards the shape of the sensors, the material of the thick piezo-resistive layer and the deposition and curing processes. However, those approaches have not proven satisfactory.
The object of the invention is to eliminate the errors due to this thermal hysteresis effect.